Magnesium stearate is a mixture of magnesium salts of
different fatty acids consisting mainly of stearic acid [(C17H35COO)2Mg;
Mr 591.3] and palmitic acid [(C15H31COO)2 Mg;
Mr 535.1] with minor proportions of other fatty acids. It contains not
less than 4.0 per cent and not more than 5.0 per cent of Mg (Ar 24.30),
calculated with reference to the dried substance. The fatty acid fraction contains not
less than 40.0 per cent of stearic acid and the sum of stearic acid and palmitic acid is
not less than 90.0 per cent.

Characters

A white, very fine, light powder, greasy to the touch,
practically insoluble in water and in ethanol.

Identification

First identification

C, D.

Second identification

A, B, D.

A. The residue obtained in the preparation of solution S (see
Tests) has a freezing point (2.2.18) not lower than 53C.

B. The acid value of the fatty acids (2.5.1) is 195 to
210, determined on 0.200 g of the residue obtained in the preparation of solution S
dissolved in 25 ml of the prescribed mixture of solvents.

C. Examine the chromatograms obtained in the test for fatty
acid composition. The retention times of the principal peaks in the chromatogram obtained
with the test solution are approximately the same as those of the principal peaks in the
chromatogram obtained with the reference solution.

To 5.0 g add 50 ml of peroxide-free etherR, 20 ml of dilute nitric acid R and 20
ml of distilled waterR and heat under
a reflux condenser until dissolution is complete. Allow to cool. In a separating funnel,
separate the aqueous layer and shake the ether layer with 2 quantities, each of 4 ml, of distilled
waterR. Combine the aqueous layers, wash
with 15 ml of peroxide-free etherR
and dilute to 50 ml with distilled waterR
(solution S). Evaporate the organic layer to dryness and dry the residue at 100-105C.
Keep the residue for identification tests A and B.

Acidity or alkalinity

To 1.0 g add 20 ml of carbon dioxide-free waterR and boil for 1 min with continuous shaking.
Cool and filter. To 10 ml of the filtrate add 0.05 ml of bromothymol blue solution R1.
Not more than 0.5 ml of 0.01M hydrochloric
acid or 0.01M sodium
hydroxide is required to change the colour of the indicator.

Chlorides (2.4.4)

0.5 ml of solution S diluted to 15 ml with waterR complies with the limit test for chlorides
(0.1 per cent).

Sulphates (2.4.13)

0.3 ml of solution S diluted to 15 ml with distilled waterR complies with the limit test for sulphates
(0.5 per cent).

Cadmium

Not more than 3 ppm of Cd, determined by atomic absorption
spectrometry (2.2.23, Method II).

Test solution

Place 50.0 mg of the substance to be examined in a
polytetrafluoroethylene digestion bomb and add 0.5 ml of a mixture of 1 volume of hydrochloric
acid R and 5 volumes of cadmium- and lead-free nitric acid R. Allow to digest
at 170C for 5 h. Allow to cool. Dissolve the residue in waterR and dilute to 5.0 ml with the same solvent.

Measure the absorbance at 232.0 nm, using a nickel
hollow-cathode lamp as a source of radiation and a graphite furnace as atomic generator.

Loss on drying (2.2.32)

Not more than 6.0 per cent, determined on 1.000 g by drying in
an oven at 100-105C.

Microbial contamination

Total viable aerobic count (2.6.12) not more than 103
micro-organisms per gram, determined by plate count. It complies with the test for Escherichia
coli (2.6.13).

Assay

Magnesium

To 0.500 g in a 250 ml conical flask add 50 ml of a mixture of
equal volumes of butanolR and ethanolR, 5 ml of concentrated ammonia R, 3 ml
of ammonium chloride buffer solution pH 10.0 R, 30.0 ml of 0.1M sodium edetate and 15 mg of
mordant black 11 triturate R. Heat to 45-50C until the solution is clearand titrate with 0.1M zinc
sulphate until the colour changes from blue to violet. Carry out a
blank titration.

1 ml of 0.1M sodium
edetate is equivalent to 2.431 mg of Mg.

Fatty acid composition

Examine by gas chromatography (2.2.28).

Test solution

In a conical flask fitted with a reflux condenser, dissolve
0.10 g of the substance to be examined in 5 ml of boron trifluoride-methanol solution R.
Boil under a reflux condenser for 10 min. Add 4 ml of heptane R through the
condenser and boil again under a reflux condenser for 10 min. Allow to cool. Add 20 ml of
a saturated sodium chloride solution R. Shake and allow the layers to separate.
Remove about 2 ml of the organic layer and dry over 0.2 g of anhydrous sodium sulphate
R. Dilute 1.0 ml of the solution to 10.0 ml with heptane R.

Reference solution

Prepare the reference solution in the same manner as the test
solution using 50.0 mg of palmitic acid CRS and 50.0 mg of stearic acid CRS
instead of magnesium stearate.

helium for chromatography R as the
carrier gas at a flow rate of 2.4 ml/min,

a flame-ionisation detector,

with the following temperature programme:

Inject 1 ml of the reference solution. When the chromatogram
is recorded in the prescribed conditions, the retention time of methyl palmitate relative
to that of methyl stearate is about 0.88. The test is not valid unless, in the
chromatogram obtained with the reference solution, the resolution between the peaks
corresponding to methyl stearate and methyl palmitate is at least 5.0.

Inject 1 ml of the test solution. Calculate the percentage
content of stearic acid and palmitic acid from the areas of the peaks in the chromatogram
obtained with the test solution by the normalisation procedure, disregarding the peak due
to the solvent.